The desirability and effectiveness of well logging systems where information is sensed in the well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Mud pulse telemetry systems provide the driller at the surface with means for quickly determining various kinds of downhole information, most particularly information about the location, orientation and direction of the drill string at the bottom of the well in a directional drilling operation. During normal drilling operations, a continuous column of mud is circulating within the drill string from the surface of the well to the drilling bit at the bottom of the well and then back to the surface. Mud pulse telemetry repeatedly restricts the flow of mud to generate a pressure increase measured at surface directly proportional to the flow restriction downhole to propagate pressure signals encoding data generated by downhole sensors through the mud upward to the surface.
A telemetry system may be lowered on a wireline located within the drill string, but is usually formed as an integral part of a special drill collar inserted into the drill string near the drilling bit. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through a downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well. The information sensed by instrumentation in the vicinity of the drilling bit is encoded into a digital formatted signal and is transmitted by instructions to pulse the mud by intermittently actuating the telemetry valve, which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface where the pulses are detected and transformed into electrical signals which can be decoded and processed to reveal transmitted information.
Representative examples of previous mud pulse telemetry systems are described in U.S. Pat. Nos. 3,949,354; 3,958,217; 4,216,536; 4,401,134; and 4,515,225, each of which is incorporated herein by reference in its entirety.
Representative samples of mud pulse generators may be found in U.S. Pat. Nos. 4,386,422; 4,699,352; 5,103,420; and 5,787,052, each of which is incorporated herein by reference in its entirety.
A servo-based actuator for a downhole pulser is described in U.S. Pat. Nos. 8,203,908 and 7,564,741, each of which is incorporated herein by reference in its entirety. A rotary pulser is described in U.S. Pat. No. 7,719,439, incorporated herein by reference in entirety.
A telemetry system capable of performing the desired function with minimal control energy is desirable, since the systems are typically powered by finite-storage batteries. One such example is found in U.S. Pat. No. 5,333,686 (incorporated herein by reference in its entirety), which describes a mud pulser having a main valve biased against a narrowed portion of the mud flowpath to restrict the flow of mud, with periodic actuation of the main valve to allow mud to temporarily flow freely within the flowpath. The main valve is actuated by a pilot valve that can be moved with minimal force. The pilot valve additionally provides for pressure equalization, thereby increasing the life of downhole batteries.
Another example of an energy-efficient mud pulser is described in U.S. Pat. No. 6,016,288 (incorporated herein by reference in its entirety), the mud pulser has a DC motor electrically powered to drive a planetary gear which in turn powers a threaded drive shaft, mounted in a bearing assembly to rotate a ball nut lead screw. The rotating threaded shaft lifts the lead screw, which is attached to the pilot valve.
Stepper motors have been used in mud pulsing systems, specifically, in negative pulse systems (see for example U.S. Pat. No. 5,115,415, incorporated herein by reference in its entirety). The use of a stepper motor to directly control the main pulse valve, however, requires a large amount of electrical power, possibly requiring a turbine generator to supply adequate power to operate the system for any length of time downhole. Such systems also require complicated electronics to commutate the motors.